The use of tyrosine kinase inhibitors, such as imatinib, against the chronic myeloid leukemia (CML)-causing kinase BCR::ABL1 has become the model for successful targeted therapy. Nevertheless, drug resistance remains a clinical problem. Analysis of genome-wide expression and genetic aberrations of an imatinib-resistant CML cell line revealed downregulation of Bruton's tyrosine kinase (), predominantly associated with B cell malignancies, and a novel kinase domain variant in imatinib resistance. This raised the question of the role of in imatinib-resistant CML. In the present study, downregulation and the presence of the variant c.1699_1700delinsAG p.(Glu567Arg) were confirmed in imatinib resistance . Similarly, BTK inhibition or small interfering RNA-mediated knockdown reduced imatinib susceptibility by 84 and 71%, respectively. overexpression was detrimental to CML cells, as proliferation was significantly reduced by 20.5% under imatinib treatment. In addition, rescue in imatinib-resistant cells restored imatinib sensitivity. The presence of the p.(Glu567Arg) variant increased cell numbers (57%) and proliferation (37%) under imatinib exposure. These data demonstrate that is important for the development of imatinib resistance in CML: Its presence increased drug response, while its absence promotes imatinib resistance. Moreover, the p.(Glu567Arg) variant abrogates imatinib sensitivity. These findings demonstrate a context-dependent role for as an oncogene in B cell malignancies, but as a tumor suppressor in other neoplasms.